Eddy current analyzers have long been used for measuring metal thickness. Earlier examples of such apparatus are disclosed in U.S. Pat. Nos. 2,862,178 to Moore (Nov. 25, 1958) and 4,553,095 to Schenk, et al. (Nov. 12, 1985). In eddy current analyzers, a probe is included which has a coil embedded therein. The coil becomes energized so as to create a magnetic field which permeates a metal structure contacted by the probe. The magnetic field induces eddy currents in an article undergoing test. The impedance reflected from the tested article de-tunes the coil in the probe and this is capable of being measured by the analyzer. Typically, eddy current analyzers are employed to gauge the thickness of a metal object by calibrating the analyzer with a known standard object having a reference thickness.
In many applications, such as in the aerospace industry, it is often necessary to drill holes in a cover plate which is to be secured to a substructure. However, in order to ensure that fastener holes will not be drilled too close to the edge of the substructure, a number of techniques are employed to draw a line on the cover plate indicating where the underlying substructure edge is located. These techniques often fall prey to inaccuracies when a line is to be drawn on a cover plate resting against a substructure hidden from view. Predrawn lines on the cover plate also present difficulties since they do not often match the actual location of an underlying substructure edge. Thus, holes may be drilled through the cover plate and substructure at points too close to the edge of the substructure. The result is a lack of structural integrity which could cause failure of an assembled structure. Accordingly, it would be highly desirable to perfect a means for detecting the actual location of a substructure edge so that it could be realistically and accurately marked on a cover plate thereby guiding the drilling operation for holes through the cover plate and substructure. Since an eddy current analyzer is capable of measuring metal materials in a non-destructive fashion, it would be of great advantage to adapt current eddy current analyzers for locating substructure edges so that the edge might be mapped on a cover plate.